Petroleum is a natural resource composed primarily of hydrocarbons. Extracting petroleum oil from the earth is expensive, dangerous, and often at the expense of the environment. Furthermore, world wide reservoirs of oil are dwindling rapidly. Costs also accumulate due to the transportation and processing required to convert petroleum oil into usable fuels such as gasoline and jet fuel.
Algae have gained a significant importance in recent years given their ability to produce lipids, which can be used to produce sustainable biofuel. This ability can be exploited to produce renewable fuels, reduce global climate change, and treat wastewater. Algae's superiority as a biofuel feedstock arises from a variety of factors, including high per-acre productivity compared to typical terrestrial oil crop plants, non-food based feedstock resources, use of otherwise non-productive, non-arable land, utilization of a wide variety of water sources (fresh, brackish, saline, and wastewater), production of both biofuels and valuable co-products such as carotenoids and chlorophyll.
Several thousand species of algae have been screened and studied for lipid production worldwide over the past several decades. Of these, about 300 species rich in lipid production have been identified. The lipid composition and content vary at different stages of the life cycle and are affected by environmental and culture conditions. The strategies and approaches for extraction are rather different depending on individual algal species/strains employed because of the considerable variability in biochemical composition and the physical properties of the algae cell wall. Conventional physical extraction processes, such as extrusion, do not work well with algae given the thickness of the cell wall and the small size (about 2 to about 20 nm) of algal cells. Furthermore, the large amounts of polar lipids in algal oil, as compared to the typical oil recovered from seeds, lead to refining issues.
Upon harvesting, typical algal concentrations in cultures range from about 0.1-1.0% (w/v). This means that as much as 1000 times the amount of water per unit weight of algae must be removed before attempting oil extraction. Currently, existing oil extraction methods for oleaginous materials strictly require almost completely dry feed to improve the yield and quality of the oil extracted. Due to the amount of energy required to heat the algal mass to dry it sufficiently, the algal feed to biofuel process is rendered uneconomical. Typically, the feed is extruded or flaked at high temperatures to enhance the extraction. These steps may not work with the existing equipment due to the single cell micrometric nature of algae. Furthermore, algal oil is very unstable due to the presence of double bonded long chain fatty acids. The high temperatures used in conventional extraction methods cause degradation of the oil, thereby increasing the costs of such methods.
It is known in the art to extract oil from dried algal mass by using hexane as a solvent. This process is energy intensive. The use of heat to dry and hexane to extract produces product of lower quality as this type of processing causes lipid and protein degradation.
Algal oil extraction can be classified into two types: disruptive or non-disruptive methods.
Disruptive methods involve lysing cells by mechanical, thermal, enzymatic or chemical methods. Most disruptive methods result in emulsions, requiring an expensive cleanup process. Algal oils contain a large percentage of polar lipids and proteins which enhance the emulsification of the neutral lipids. The emulsification is further stabilized by the nutrient and salt components left in the solution. The emulsion is a complex mixture, containing neutral lipids, polar lipids, proteins, and other algal products, which extensive refining processes to isolate the neutral lipids, which are the feed that is converted into biofuel.
Non-disruptive methods provide low yields. Milking is the use of solvents or chemicals to extract lipids from a growing algal culture. While sometimes used to extract algal products, milking may not work with some species of algae due to solvent toxicity and cell wall disruption. This complication makes the development of a generic process difficult. Furthermore, the volumes of solvents required would be astronomical due to the maximum attainable concentration of the solvent in the medium.
Accordingly, to overcome these deficiencies, there is a need in the art for improved methods and systems for extraction and fractionating algal products, in particular algal oil, algal proteins, and algal carotenoids. This process can be further improved by introducing a highly non-polar solvent to the extraction system, thereby avoiding the cost of evaporating and recycling all of the solvent used for extraction.